A radio communications system may be largely categorized into an FDD (Frequency Division Duplex) scheme and a TDD (Time Division Duplex) scheme.
According to the FDD scheme, an uplink transmission and a downlink transmission are performed through different frequency bands. According to the TDD scheme, an uplink transmission and a downlink transmission are performed through the same frequency band and at different times. According to the TDD scheme, channel responses are substantially reciprocal to each other. This means that a downlink channel response and an uplink channel response are almost the same in a given frequency region. Accordingly, in a TDD-based radio communications system, a downlink channel response can be acquired from an uplink channel response.
According to the TDD method, an entire frequency band undergoes time division into an uplink transmission and a downlink transmission. Therefore, a downlink transmission by a base station (BS) and an uplink transmission by a mobile station (MS) cannot be simultaneously performed. In a TDD system where an uplink transmission and a downlink transmission are differentiated from each other in the unit of subframes, an uplink transmission and a downlink transmission are performed on different subframes.
A radio communications system includes a base station (BS) which provides a service to neighboring cells. Generally, a terminal or a mobile station (MS) can communicate with the BS when being within the service coverage of the BS. However, when there is an obstacle such as a building or when the terminal or the MS is positioned at a cell boundary, the MS cannot perform a communication with the BS or can perform a communication with an inferior communication quality.
In order to extend the service coverage of the BS, there have been proposed various methods.
One of the various methods is to introduce a relay station (RS) to the radio communications system. The RS is operated as an intermediary between the BS and the MS (or between two MSs and between MS/BS and another RS). More concretely, the RS allows data to be transferred between the BS and the MS far therefrom through two hops or multi hops, not through a single link for direct transfer. This RS may extend the service coverage of the BS, and may enhance a cell boundary performance. Furthermore, the RS may enhance a cell throughput.
The RS was firstly developed from a time division duplex (TDD) radio communications system such as Mobile WiMAX (e.g., IEEE 802.16j/m).
In order to enhance the performance, a Frequency Division Duplex (FDD) radio communications system has started to research about the introduction of a relay station. The FDD radio communications system may include an FDD-based 3GPP (Generation Project Partnership) LTE (Long Term Evolution) system, or a Mobile WiMAX system for supporting FDD, etc.
FIG. 1 illustrates a radio communications system using a relay station.
As shown, the radio communications system includes one or more base stations (BS) 21, 22 and 23.
Each base station 21, 22 and 23 provides a communications service to a specific geographical area (cell) 21a, 22a and 23a. The cell may be divided into a plurality of areas (sectors). One base station may include one or more cells.
The base stations 21, 22 and 23 indicate fixed stations communicating with terminals 11, 12 and 13, and may be called eNB (evolved-NodeB), BTS (Base Transceiver System), Access Point, AN (Access Network), etc.
Hereinafter, a downlink (DL) indicates a communication to a terminal from a base station, and an uplink (UL) indicates a communication from a terminal to a base station. In the DL, a transmitter may be part of a base station, and a receiver may be part of a terminal. In the UL, a transmitter may be part of a terminal, and a receiver may be part of a base station.
In uplink transmission, the terminal 11 is operated as a source station, and transmits data to the base station serving as a destination station. In downlink transmission, the base station 21 is operated as a source station, and transmits data to the terminal 11 serving as a destination station.
As shown, the radio communications system may include one or more relay stations 31, 32 and 33.
As shown, the relay stations are positioned on an outer periphery area or a shadow area of a cell, and relays data between the base station and the terminal. Here, the base station performs functions such as connectivity, management, control and resource allocations between the relay station and the terminal.
Referring to FIG. 2, the base station performs a communication with the terminal through the relay station.
As shown, the relay station 31 relays the UL and the DL.
In UL transmission, the terminal 11 serving as a source station o transmits UL data to a destination station, i.e., the base station 21 and the relay station 31. Then, the relay station 31 relays the UL data of the terminal 11 to the base station 21.
In DL transmission, the base station 21 serving as a source station transmits DL data to a destination source, i.e., the terminal 11 and the relay station 31. Then, the relay station 31 relays data from the source station (the base station 21) to the destination station (the terminal 11).
As shown, the relay station may be implemented in one or plurality in number. That is, the relay stations 32 and 33 may exist between the base station and the terminal 12.
The relay station may adopt a relaying scheme such as an AF (amplify and forward) scheme and a DF (decode and forward) scheme.
FIGS. 3 and 4 illustrate influences by interference due to the introduction of a relay.
As can be seen from FIG. 3A, the relay station 30 is connected to the first base station 21. The first terminal 11 is placed within the coverage of the first base station 21, and performs communications with the first base station 21. And, the second terminal 12 is placed within the coverage of the relay station 30, and performs communications with the relay station 30.
When the relay station 30 connected to the first base station 21 transmits DL data to the second terminal 12, the DL from the first base station 21 to the first terminal 11 may be interfered.
As can be seen from FIG. 3B, the first base station 21 and the second base station 22 are adjacent to each other. And, the relay station 30 is positioned on an outer periphery region of the coverage of the first base station 21. In the case that the first base station 21 transmits backhaul data to the relay station in the DL, the first base station 21 transmits the backhaul data with high transmission power since the relay station 30 is positioned on the outer periphery region. Here, the DL from the second base station 22 to the second terminal 12 is interfered by the backhaul data transmitted with high transmission power.
Referring to FIG. 4, the first relay station 31 and the second relay station 32 are connected to the first base station 21. The first terminal 11 is connected to the first relay station 31, and the second terminal 32 is connected to the second relay station.
If the first base station 21 transmits backhaul data to the first relay station 31 in the DL, the DL from the second relay station 32 to the second terminal 12 may be interfered. On the contrary, if the second relay station 32 transmits backhaul data to the second terminal in the DL, the DL from the first base station 21 to the first relay station 31 may be interfered.